This invention relates to red fluorescent materials that can be used as dopants in organic electroluminescent devices.
One of the applications for using red fluorescent materials is in organic EL (electroluminescent) devices. Organic EL devices are known to be highly efficient and are capable of producing a wide range of colors. Useful applications such as flat-panel displays have been contemplated and commercialization is well underway. Representative of earlier organic EL devices are Gurnee et al., U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee, U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, xe2x80x9cDouble Injection Electroluminescence in Anthracene,xe2x80x9d RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner,. U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. Typical organic emitting materials were formed of a conjugated organic host material and a conjugated organic activating agent having condensed benzene rings. The organic emitting material was present as a single layer medium having a thickness much above 1 micrometer. Thus, this organic EL medium was highly resistive and the EL device required a relatively high voltage ( greater than 100 volts) to operate.
The most recent discoveries in the art of organic EL device construction have resulted in devices having the organic EL medium consisting of extremely thin layers ( less than 1.0 micrometer in combined thickness) separating the anode and cathode. Herein, the organic EL medium is defined as the organic composition between the anode and cathode electrodes. Typically, organic EL devices are composed of three layers of organic molecules that are interposed between a transparent electrode and a metallic electrode. The three layers include an electron-transporting layer, a luminescent layer and a hole-transporting layer. The luminescent layer between the electron-transporting and hole-transporting layers provides an efficient site for the recombination of the injected hole-electron pair and resultant electroluminescence. A hole-injecting layer may be added to reduce the driving voltage. Optionally a hole- or electron-blocking layer may be added to improve the luminance efficiency. An organic EL device comprising four to six layers of organic molecules is thus obtained.
The extremely thin organic EL medium offers reduced resistance, permitting higher current densities for a given level of electrical bias voltage. Since light emission is directly related to current density through the organic EL medium, the thin layers coupled with increased charge injection and transport efficiencies have allowed acceptable light emission levels to be achieved with low applied voltages in ranges compatible with integrated circuit drivers, such as field effect transistors.
Organic EL devices have lower driving voltages and have been shown applicable for use in full-color flat-panel displays. The investigation in organic EL devices and materials has attracted a lot of worldwide attention and investment. The improvements in organic EL devices such as color, stability, efficiency and fabrication methods have been disclosed in U.S. Pat. Nos.: 5,151,629; 5,150,006; 5,141,671; 5,073,446; 5,061,569; 5,059,862; 5,059,861; 5,047,687; 5,104,740; 5,227,252; 5,256,945; 5,069,975; 5,122,711; 5,366,811; 5,126,214; 5,142,343; 5,389,444; 5,458,977; 5,908,581; 5,935,720; 6,091,195.
One of the ultimate objectives of the developments in organic light-emitting diodes (OLED) is the application in full color flat-panel displays. Therefore, obtaining three primary colors of red, green and blue that meet commercial requirements is critical in the product applications of OLED. In current developments, one of the most commonly used methods to modify the color and luminance efficiency is by doping a small amount of a highly fluorescent material into the main EL emitter as a dopant. In this manner, lights of three primary colors of red, green and blue can be obtained. Accordingly, seeking perfect doping materials of red, green and blue (RGB) and improving the luminous efficiency, luminance and chromaticity of the RGB colors have become one of the most important research objectives. Among the three primary colors of lights, the research for a primary red light is most problematic.
The organic EL mechanism utilizes the energy transfer between the host and guest materials. For example, a red organic EL emission can be obtained by doping a small amount of a red fluorescent material (a guest) into tris-(8-hydroxyquinolinato)aluminum (Alq3) (the host) to transfer the green luminescent energy of Alq3 to the red guest (see U.S. Pat. No. 4,769,292). In the current red fluorescent dopants, a series of materials developed by Eastman Kodak Company of the U.S. are most notable (see U.S. Pat. No. 5,908,581). Alq3 is a suitable host for red EL emitters since its emission at 530 nm is adequate to sensitize guest EL emission in the red spectral region.
The photoluminescent efficiency of the earliest red guest dye, 4-(dicyano-methylene)-2-methyl-6-(p-dimethylaminostyryl)4-pyran (DCM), (Formula A) is 78% at xcexmax=596 nm in a reasonable doping concentration (about 0.5%). The light emitted is a yellow-biased red. Subsequently, Kodak developed a julolidyl derivative DCJ red guest material (Formula B) which emits light with a peak wavelength within the red-range of 610xcx9c690 nm. The Alq3-based EL emitter has a maximum luminous intensity when doped with 0.57% of DCJ. However, the color of the light emitted is reddish orange due to the green luminescence of Alq3 that was not totally quenched. While more saturated red luminescence can be obtained when DCJ doped into the emitter is  greater than 4% of the emitter, the luminance efficiency drops to 50% of that with the maximum luminous intensity. This is because the red guest dyes aggregate and influence each other at high concentrations, which leads to the quenching of the luminescence. 
Other considerations in the fields of fluorescence and electroluminescence applications are the purity of fluorescent materials and the degree of synthetic complexities, including consideration of yield loss due to post-process purification procedures. In the aforementioned patents, the preparation and subsequent purification of both DCM and DCJ are complicated by the inevitable generation of a significant amount of an unwanted corresponding bis-condensed dye caused by the further reaction of the xe2x80x9cactivexe2x80x9d methyl group present in the fluorescent dye molecules (see Hammond, Optics Comm., 1989, 29, 331). Furthermore, once the bis-condensed dye is formed in the reaction mixture, it is difficult to be removed and tends to diminish or extinguish DCM (or DCJ) fluorescence.
To reduce the quenching of the guest dye per se and correspondingly raise the luminance efficiency, Kodak developed derivatives which are based on tetra-methyljulolidine (DCJT) (Formula C) (see Chen et al., Proc. 2nd Internat. Sym. Chem. Functional Dyes. 1992, 536). They have four more methyl groups than DCJ. Thereby, the methyl groups result in a steric hindrance between dye molecules, and thus reduce the quenching effect caused by the formation of aggregate due to high concentration.
While DCJT has a good luminance efficiency, the controlling of synthesis and purification remains a problem. The reactive methyl group at the C-6 position of pyran moiety of the DCJT product may further react with the starting material tetramethyljulolidyl aldehyde to produce the by-product, bis-DCJT (Formula D). (See C. H. Chen, C. W. Tang, J. Shi and K. P. Klubek, Macromol. Symp. 125, 49 1997.) Therefore, the preparation and subsequent purification of DCJT are also complicated by the inevitable generation of significant amount of the unwanted bis-condensed dye that tends to decrease the fluorescence efficiency of DCJT. 
To overcome this problem, C. H. Chen et al. (see U.S. Pat. No. 5,908,581, issued Jun. 1, 1999, entitled xe2x80x9cRed Organic Electroluminescent Materialsxe2x80x9d and U.S. Pat. No. 5,935,720, issued Aug. 10, 1999, entitled xe2x80x9cRed Organic Electroluminescent Devicesxe2x80x9d) synthesized what appeared to be the first asymmetric 4-(dicyanomethylene)-2-methyl-4H-pyran, from which derivatives of DCJTR can be prepared without the reactive methyl group on the C-6 position of pyran moiety (Formula E). The DCJTR derivatives include DCJTP (R=phenyl), DCJTE (R=ethyl), DCJTB (R=t-butyl), DCJTM (R=mesityl) and the like. The syntheses of those derivatives produce no (or less) bis-condensed by-products contaminant, and the purity can be above 98%. 
Since 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) has good luminance, the red luminescent devices produced by doping (1%-4% by volume) of the compound into the main EL emitters will meet the preliminary requirement of commercial applications. Currently, companies such as Kodak and Sanyo have applied the compounds to red organic EL devices.
However, the red EL devices produced by doping the main EL emitter with DCJTB have some drawbacks. The main disadvantage is that the required doping concentration of DCJTB is up to 2-4% if the hue of light emission is to be tuned to the CIE red position in standard displays. High concentrations of dopants will give rise to the quenching of fluorescence, resulting in low luminance. In particular, among most of the current OLED RGB primary colors, the luminance efficiency of red light is poor.
In addition, the yield of the key intermediate tetramethyljulolidine in the synthesis of DCJTR is low. The reaction yield depends on the types and reaction conditions of acidic catalysts (see C. H. Chen, S.-W. Wen, P. Balaganesan, Third International Conference on Electroluminescence of Molecular Materials and related phenomena, Sept. 5-8, 2001, L.A., Calif., U.S.A., Abstract: O-51).
Therefore, in the red organic electroluminescent devices, there exits the need to improve the EL performance of the fluorescent dopant material; to improve the overall yield of the red dopant material, in particular, the key intermediate, tetramethyljulolidine synthesis; and to improve the purification process of the red dopant by sublimation.
This invention replaces the four methyl groups of DCJTR with five strategically positioned aliphatic or aromatic groups with the intention to alleviate the fluorescence-quenching problem of the dye at high concentration. It is expected that the interaction between molecules of the dye will be lessened due to the enhanced steric interaction that may reduce the propensity for dye aggregation at high concentration.
The red fluorescent dopants, 4-(dicyanomethylene)-2-R6-(1,3,3,7,7-penta-methyljulolidyl-9-enyl)-4H-pyran (DCJPR, Formula I) series of compounds, according to the present invention are prepared by a new reaction route that is different from and better than that of DCJTR. In particular, the yield of the intermediate pentamethyljulolidine of DCJPR of the present invention is significantly higher than that of the intermediate tetramethyljulolidine of DCJTR. As a result, the overall reaction yield of DCJPR is far better than that of DCJTR thereby offers the advantage of producing the red dopant more cost effective.
When compared under similar doping concentrations, it was found that the new dopant DCJPB (Formula I, R=t-Bu) offers higher luminance yield than that of DCJTB at the similar chromaticity. In addition, DCJPB having a lower melting point than that of DCJTB is easier to purify by train sublimation without incurring thermal-induced decomposition.
It is another objective of the present invention to provide an improved red fluorescent material which fluoresces in the red region of the visible spectrum.
It is another objective of the present invention to provide an improved red fluorescent material which offers higher reaction yield and easier purification and mass production capabilities.
This objective is achieved by a red fluorescent material comprising a compound of formula I: 
wherein R1, R2, R3, R4, R5, and R6 each independently represents a linear or branched aliphatic group having from 1 to 10 carbon atoms or an aromatic group having from 5 to 20 carbon atoms.
This objective is also achieved by a red fluorescent material comprising a compound of formula II: 
wherein R1, R2, and R3 each independently represents a linear or branched aliphatic group having from 1 to 10 carbon atoms or an aromatic group having from 5 to 20 carbon atoms.
This objective is also achieved by a red fluorescent material comprising a compound of Formula III: 
wherein R represents a linear or branched aliphatic group having from 1 to 10 carbon atoms or an aromatic group having from 5 to 20 carbon atoms.
The red fluorescent dopants, 4-(dicyanomethylene)-2-R6-(1,3,3,7,7-penta-methyljulolidyl-9-enyl)-4H-pyran (DCJPR), are prepared by a new reaction route different from that of DCJTR. The yield of the intermediate pentarnethyl-julolidine of DCJPR of the present invention is significantly higher than that of the intermediate tetramethyljulolidine of DCJTR. Therefore, the purification and quantitative production of DCJPR are easier than that of DCJTR.
It is a feature of the present invention that when the compound is applied to EL devices it provides a fluorescent compound devoid of chemical reactivity of the extra xe2x80x9cactivexe2x80x9d methyl group, thereby avoiding the formation of the unwanted bis-condensed dye in the reaction product mixture. Moreover, in the properties of organic EL devices under the same doping concentrations, DCJPB has a thermal stability similar to DCJTB, but a better luminous efficiency and an almost same chromaticity.
When used as a dye material in other devices such as dye lasers and luminescent solar concentrators, fluorescent inks and other applications, the material is particularly useful.